Integration of laser sources and detectors for a passive optical network

ABSTRACT

Various methods and apparatuses are described in which an array of optical gain mediums capable of lasing are contained in a single integral unit. The array may contain four or more optical gain mediums capable of lasing. Each optical gain medium capable of lasing supplies a separate optical signal containing a band of wavelengths different than the other optical gain mediums capable of lasing in the array to a first multiplexer/demultiplexer. A connection for an output fiber exists to route an optical signal to and from a passive optical network.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from and is a divisionalapplication of U.S. patent application Ser. No. 11/983,720, filed Nov.9, 2007 now U.S. Pat. No. 7,593,444 which is a divisional application ofU.S. patent application Ser. No. 10/741,134, filed Dec. 19, 2003, whichhas issued as U.S. Pat. No. 7,313,157.

FIELD

Embodiments of the invention generally relate to optical networks. Moreparticularly, an aspect of an embodiment of the invention relates toarray of optical gain mediums capable of lasing contained in a singleintegral unit.

BACKGROUND

Fiber optic systems typically transmit optical signals back and forthbetween a central office to a multitude of residential and businesslocations. Each residential or business location may be assigned anarrow bandwidth of wavelengths or channel within an overall opticalsignal to communicate with and from the central office. As the number ofsubscribers using that fiber optical system increases, the amount ofcomponents in the central office may increase to transmit and receiveoptical signals from those subscribers.

SUMMARY

Various methods and apparatuses are described in which an array ofoptical gain mediums capable of lasing contained in a single integralunit. The array may contain four or more optical gain mediums capable oflasing. Each optical gain medium capable of lasing supplies a separateoptical signal containing a band of wavelengths different than the otheroptical gain mediums capable of lasing in the array to a firstmultiplexer/demultiplexer. A connection for an output fiber exists toroute an optical signal to and from a passive optical network.

Other features and advantages of the present invention will be apparentfrom the accompanying drawings and from the detailed description thatfollows below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by example and not limitation inthe figures of the accompanying drawings, in which like referencesindicate similar elements and in which:

FIG. 1 illustrates a block diagram of an embodiment of an array ofmultiple wavelength optical gain mediums capable of lasing.

FIG. 2 illustrates a block diagram of an embodiment of an array of fouror more distributed feedback lasers in an integral unit. The array offour or more distributed feedback lasers 202 each coupled to a powersplitter 212.

FIG. 3 illustrates a block diagram of an embodiment of an array oflasers and a broadband light source to supply an optical signal to eachof the lasers all contained in an integral unit.

FIG. 4 illustrates a block diagram of an embodiment of an array of fouror more optical receivers and a multiplexer/demultiplexer integrated into a single integral unit.

FIG. 5 illustrates a block diagram of an embodiment of an array ofoptical gain mediums capable of lasing and an array of optical receiverscontained within an integral unit.

FIG. 6 illustrates a block diagram of an embodiment of an array oflasers and an array of optical receivers in an integral unit.

DETAILED DISCUSSION

In general, various methods and apparatuses are described in which anarray of optical gain mediums capable of lasing and an array of opticalreceivers are contained in a single integral unit. The array may containfour or more optical gain mediums capable of lasing. Each optical gainmedium capable of lasing supplies a separate optical signal containing aband of wavelengths different than the other optical gain mediumscapable of lasing in the array to a first multiplexer/demultiplexer.Similarly, a second multiplexer/demultiplexer may route optical signalsto the array of optical receivers. A connection for an output fiberexists to route an optical signal to and from a passive optical network.

FIG. 1 illustrates a block diagram of an embodiment of an array ofmultiple wavelength optical gain mediums capable of lasing. The planarlightwave circuit 100 may contain an array of four more optical gainmediums capable of lasing 102, such as lasers, amultiplexer/demultiplexer 112, an optical amplifier 104, a broadbandwavelength reflector 106, an electrical modulation source 108, aconnection 110 to an output fiber, and an output fiber 114 going to awavelength-division-multiplexed passive optical network.

Multiple optical gain mediums capable of lasing 102, such as a firstgain medium 116 through an Nth gain medium 118, may exist in the planarlight circuit 100. Each gain medium 102 supplies an optical signalhaving a narrow band of wavelengths (λ) different than the other gainmediums. Each of the gain mediums 102 couples to it's own port on themultiplexer/demultiplexer 112. The broadband wavelength reflector 106couples to the output of the multiplexer/demultiplexer 112. Thewavelength reflector 106 routes a portion of each optical signal asregenerative feedback through the multiplexer/demultiplexer 112 to thegain medium 102 that supplied the optical signal.

The modulation source 108 may supply a data signal to the gain mediumarray 102 to directly modulate the gain mediums in that array. Theelectrical modulation source 108, by directly supplying the data to aparticular gain medium, directly modulates that gain mediums capable oflasing. For example, the modulation source 108 may directly modulate thefirst gain medium capable of lasing 116. The data signal is amplified bythe first gain medium capable of lasing 116 at a band of wavelengthsaround approximately one or more of its cavity modes. The first gainmedium capable of lasing 116 routes the modulated signal to a firstinput 120 of the multiplexer/demultiplexer 112.

The multiplexer/demultiplexer 112 routes the modulated signal to thewavelength reflector 106. The wavelength reflector 106 routes a portionof the modulated signal as regenerative feedback back through themultiplexer/demultiplexer 112 to the first gain medium capable of lasing116 that supplied that modulated signal. The modulated signal and thereflected portion of the modulated signal reinforce each other, inphase, at a resonant frequency of the first gain medium capable oflasing 116. The modulated signal and the reflected portion of themodulated signal are also amplified by the first gain medium capable oflasing 116.

The first gain medium capable of lasing 116 then transmits thereinforced modulated signal through the multiplexer/demultiplexer 112and a portion of that reinforced modulated signal passes through thewavelength reflector 106 to the connection 110 to the output fiber.Also, as described above, the wavelength reflector 106 reflects aportion of that reinforced modulated signal back through themultiplexer/demultiplexer 112 to the first gain medium. Thisregenerative amplification of a narrow band occurs for each of the gainmediums capable of lasing 102. Each of the gain mediums capable oflasing 102 amplifying its own distinct narrow band of wavelengths.

The multiplexer/demultiplexer 112 acts as a narrow band filter to definethe band of wavelength developed by the first gain medium capable oflasing 116. A natural characteristic of the multiplexer/demultiplexer112 is to pass a different band of wavelengths on each of its outputs.For example, the first output may pass the band of wavelengths from 1530to 1531 nanometers (nm). The second output may pass the band ofwavelengths from 1531 to 1532 nm. Therefore, themultiplexer/demultiplexer 112 creates a narrow band of wavelengthssupplied back to each gain medium capable of lasing 102. Accordingly,each gain medium capable of lasing 102 develops and amplifies a resonantwavelength within that band of wavelengths corresponding to a cavitymode of the gain medium. When the amplified band of wavelengths isreinforced with the reflected modulated signal, then the gain mediumgenerates an optical signal of sufficient power to transmit over thepassive optical network to a subscriber's home. The reflected modulatedsignal provides resonant feedback to the gain medium capable of lasing.

The array of gain mediums capable of lasing 102, themultiplexer/demultiplexer 112, the optical amplifier 104, and thebroadband wavelength reflector 106 can all be integrated into a singleintegrated unit. The integral unit may be a single substrate where allthe components are grown on that single substrate. Alternatively, theintegral unit may be two or more substrates made out of differentmaterials and physically joined together.

The integral unit may use optical couplings 122 other than opticalfibers in the optical path of the integral unit, such as air, lensarrays, or other such waveguides. Optical fibers typically requireminimum bend radiuses and have other disadvantages that do not allowthem to be used in a small compact space. However, optical couplings 122other than optical fibers such as air or lens arrays may be used in avery small physical space to allow the communication of optical signalsfrom one optical component to the next optical component. Further, in anintegral unit where all of the gain mediums capable of lasing 102 aregrown on the same substrate, the physical spacing between the gainmediums may be much shorter and smaller in physical size than if each ofthe gain mediums capable of lasing 102 was a fabricated as a discretecomponent and placed onto a common platform.

The substrate may be composed of Indium-Phosphide, where both activedevices, such as the gain mediums capable of lasing, optical amplifiers,and modulators, can be integrated along with passive devices, such asthe waveguides and multiplexer/demultiplexer. The substrate may also becomposed from other materials, such as erbium-doped silica.

The array of gain mediums capable of lasing 102 may contain a largenumber of lasers, such as thirty-two or sixty-four lasers, acting asgain mediums, however the gain medium array may be as small as four orso laser sources acting as gain mediums capable of lasing. The opticalgain medium capable of lasing may be a distributed feed back laserhaving its center wavelength set by a Bragg grating, a Fabry Perot laserdiode, reflective semiconductor optical amplifiers, or similar lasergrown on a single substrate. Each of the gain mediums capable of lasinghas its own resonant wavelength and may be biased to operate above orbelow a lasing threshold.

The reflective semiconductor optical amplifiers may be gain mediumscapable of lasing that have a highly reflective back facet, such as 90%,with a front facet surface that is at a non-normalangle/non-perpendicular angle to the optical waveguide of the reflectivesemiconductor optical amplifier. The highly reflective back facet causesa greater amount of the injected wavelengths to be amplified andreflected back out of the reflective semiconductor optical amplifier.The front facet waveguide at a non-normal angle reduces the front facetreflectivity and allows a greater amount of gain to be provided by thereflective semiconductor optical amplifier before lasing action occursin the reflective semiconductor optical amplifier on the injectedwavelengths.

The optical amplifier 104 coupled to the multiplexer/demultiplexer 112may amplify the optical signal coming from the multiplexer/demultiplexer112 to increase the overall gain and compensate for any insertionlosses. A connection 110 to an output fiber exists in the output opticalpath of the multiplexer/demultiplexer 112 to a passive optical network.The passive optical network may have an optical splitting component suchas a wave division multiplexer.

The gain mediums capable of lasing may be also continuous wave sourcesmodulated by a separate array of modulators rather than directlymodulated. Each continuous wave modulator connects to its own gainmedium. The continuous wave modulator data modulates the continuous wavecoming from the gain medium capable of lasing. Themultiplexer/demultiplexer 112 may be an array wave-guide, an eschellegrating, or other similar technique to combine multiple uniquewavelengths into a single wave-guide with a low signal power loss.

The wavelength reflector 106 may also be located at the output of theplanar lightwave circuit 100 to provide regenerative optical feedback toeach gain medium capable of lasing 102 in order to develop the resonantwavelength of that gain medium. The wavelength reflector 106 may becreated by etching a vertical facet in the wave guide to create a changein the index of refraction, or be a Bragg grating, or maybe a coating atthe edge of the substrate of the integral unit/interface with the outputoptical fiber with a reflective material to reflect a portion of theoptical signal back to the multiplexer/demultiplexer 112, or may be agrating at the entrance of the optical fiber 114 pigtailed to the planarlightwave circuit 100.

The construction of the planar lightwave circuit 100 having an array ofgain mediums capable of lasing resembles a distributed laser having anexternal cavity which can operate above or below the lasing threshold inorder to develop or reinforce a multiple bands of wavelengths eachhaving different wavelength bands. Thus, the construction of thedistributed laser may be defined as from the gain mediums capable oflasing section through the multiplexer/demultiplexer 112 to thewavelength reflector 106 and back through the multiplexer/demultiplexer112 to each gain medium section. Each laser in the gain medium sectionmay or may not have a reflective front facet.

FIG. 2 illustrates a block diagram of an embodiment of an array of fouror more distributed feedback lasers in an integral unit. The array offour or more distributed feedback lasers 202 each coupled to a powersplitter 212. Each distributed feedback laser in the array 202 suppliesa separate optical signal containing a band of wavelengths differentthan the other distributed feedback lasers in that array to the powersplitter. Each distributed feedback laser in the array 202 has it'scenter wavelength of that band of wavelengths set by a Bragg grating atthe output of the distributed feedback laser. For example, the firstdistributed feedback laser 216 has the center wavelength of the band ofwavelengths supplied from the laser set by the first Bragg grating 224intermixed with the gain medium of the first distributed feedback laser216. A semiconductor optical amplifier 204 may exist in an outputoptical path of the power splitter 212 to make up for the insertionlosses caused by the power splitter 212. A connection 210 exists for anoutput fiber to route the optical signal from the power splitter 212 orat least in the output optical path of the power splitter 212 to awave-division-multiplexed passive optical network. The array ofdistributed feedback lasers 202 may be integrated onto a first substrate226. The semiconductor optical amplifier 204, the power splitter 212,and the connection 210 may be integrated into a second substrate 228that is joined to the first substrate 226 and that communicates opticalsignals in the optical path between the first substrate and the secondsubstrate using optical couplings 222 such as air or lens arrays but notusing optical fibers.

Thus, the active components may be fabricated on a first substrate 226and the passive components may be fabricated on a second substrate 228that are merged and physically joined together into an integral unit.The first substrate 226 as discussed may be silicon dioxide, indiumphosphide, or similar substrate. Note, Distributed Bragg Reflectorlasers, for example, can also be used to generate the optical signalcontaining the band of wavelengths instead of the distributed feedbacklasers. The Distributed Bragg Reflector lasers may have Bragg grating atthe output of the laser to set the center wavelength of that laser.

FIG. 3 illustrates a block diagram of an embodiment of an array oflasers and a broadband light source to supply an optical signal to eachof the lasers all contained in an integral unit. The integral unit maycontain an array of four or more lasers such as Fabry-Perot laser diodes302 on a first substrate 326. The integral unit may contain a secondsubstrate 328 containing a multiplexer/demultiplexer 312, a broadbandlight source 330, and a connection 310.

The broadband light source 330 supplies an optical signal containing abroad band of wavelengths, such as the C-band (1530 nm˜1560 nm), throughan optical coupler 331 to the multiplexer/demultiplexer 312. Each of theFabry-Perot laser diodes in the array 302 couples to it's own port onthe multiplexer/demultiplexer 312. Each of the Fabry-Perot laser diodesin the array 302 receives a spectral slice of the optical signal fromthe broadband light source 330 in order to wavelength lock an outputwavelength of that Fabry-Perot laser diode to within the bandwidth ofthe injected spectral slice. For example, the first Fabry-Perot laserdiode 316 may receive a spectral slice of 1530 to 1531 nm. The firstFabry-Perot laser diode 316 then may reflect and amplify the spectralslice back out through the multiplexer/demultiplexer 312 to theconnection 310. The connection 310 couples to an output fiber 314 inorder to route an optical signal to the wavelength division multiplexingpassive optical network.

All of the Fabry-Perot laser diodes in the array 302, themultiplexer/demultiplexer 312, the connection 310, and the broadbandlight source 330 are integrated into a compact integral unit. Thebroadband light source 330 may also be discrete from the integral unit.The broadband light source 330 may consist of two or more superluminescent diodes connected to supply orthogonal polarized signals, anerbium fiber that acts as a broadband light source, an erbium dopedwaveguide, a single super luminescent diodes connected to the integralunit with polarization persevering fiber, a single on chip superluminescent diode, or other similar light emitting source. All of thecomponents may be located in a single planar lightwave circuit.

FIG. 4 illustrates a block diagram of an embodiment of an array of fouror more optical receivers and a multiplexer/demultiplexer integrated into a single integral unit. The integral unit may contain an array offour or more optical receivers 432, such as a first optical receiver 434through an Nth optical receiver 436, an electrical processing chip 438to process the received data signals λ1 through λn, amultiplexer/demultiplexer 430, and a connection 410 to receive an inputfiber from a wave-division-multiplexed passive optical network having acomponent to combine multiple optical signals coming from subscribers ofthat passive optical network. Each of the optical receivers in the array432 may contain one or more photo detectors.

The integral unit containing the receivers 440 may be located at thecentral office where returning signals need to be locally processed.Note, the integral unit of the optical gain mediums capable of lasingmay also be located at the central office where minimizing the spaceoccupied by components is at a premium and all of the components may becompactly, centrally located. The first substrate 426 having the opticalreceivers array 432 may be composed of indium phosphide, galliumarsenide, silicon, or other similar semiconductor substrates. The firstsubstrate 426 may be coupled in a planar lightwave circuit to themultiplexer/demultiplexer 430 on the second substrate 428. Theelectrical processing chip 438 containing the electrical processingcomponents that process the signal from the optical receivers in thearray 432 can also be on another third substrate 442 made of silicon.The third substrate 442 may be coupled and physically joined to thefirst substrate 426. The substrates 426, 428 in the integral unit maycommunicate optical signals via wave-guides without optical fibers, suchas the first non-optical fiber waveguide 422, the second non-opticalfiber waveguide 423, and the third non-optical fiber waveguide 425. Allof the substrates 426, 428, 442 may be fabricated as a single integralunit 440.

FIG. 5 illustrates a block diagram of an embodiment of an array ofoptical gain mediums capable of lasing and an array of optical receiverscontained within an integral unit. The array of optical gain mediumscapable of lasing 550 may generate N number of individual bands ofwavelengths. Each optical gain medium capable of lasing communicates anoptical signal across the passive optical network to a correspondingsubscriber, such as the first subscriber location 552. The array ofoptical receivers 554 may receive N number of individual bands ofwavelengths from those subscribers. For example, a first receiver mayreceive an optical signal generated from the first subscriber location552. The array of optical receivers 554 may include the same number ofreceivers as gain mediums capable of lasing in the array of gain mediumscapable of lasing. For example, a first array of optical receivers 554may contain thirty-two receivers and a first array of gain mediumscapable of lasing may contain thirty-two gain mediums capable of lasing.The array of optical gain mediums capable of lasing 550 and the array ofoptical receivers 554 may be on a single substrate or may be on separatesubstrates joined to each other in the integral unit 556.

Each of the arrays 550, 554 may contain a multiplexer/demultiplexer or apower splitter to distribute the signals coming from and going to thepassive optical network 558. On each of the arrays 550, 554, thecomponents may be grown on that substrate to make the spacing betweenindividual components as small as possible. The integral unit 556 mayalso contain a band splitting filter 560 and a broadband light source.The broadband light source 562 may also be exterior to the integral unit556.

The broadband light source supplies an optical signal containing abroadband of wavelengths, such as the L-band, to themultiplexer/demultiplexer in the optical gain medium array 550. Asdiscussed above, the multiplexer/demultiplexer routes a narrow bandoptical signals to each of the optical gain medium in the array 550 towavelength lock the output wavelength of the optical gain medium capableof lasing within the bandwidth of the injected spectral slice.

The array of optical gain mediums capable of lasing 550 through itsmultiplexer/demultiplexer may send a single optical signal consistingof, for example, thirty-two individual bands of wavelengths containedwithin the C-band across the passive optical network 558 to a remotemultiplexer/demultiplexer 564. The remote multiplexer/demultiplexer 564may distribute the individual band of wavelengths from each optical gainmediums capable of lasing in the array 550 to a corresponding subscriberlocation. For example, the remote multiplexer/demultiplexer 564 maydistribute the band of wavelengths from the second optical gain mediumcapable of lasing to the location of a second subscriber 566. The remotemultiplexer/demultiplexer 564 may distribute all of the N number ofindividual band of wavelengths from the array of optical gain mediumscapable of lasing 550 in this manner to corresponding subscriberlocations.

The group of users/subscribers may also transmit optical signals back tothe array of receivers 554 in central office in the L-band (1570 nm˜1600nm). The band-splitting filter 560 separates the L-band wavelengths fromthe C-band wavelengths. The band-splitting filter 560 routes the L-bandsignals to the array of optical receivers 554 and the C-band wavelengthsfrom the broad light source to the array of optical gain mediums capableof lasing 550.

The transmitters in the central officer may use a first band such as theL-band to communicate information to subscribers and the transmitters atthe subscribers use another band such as the C-band to communicateinformation to the central office. Accordingly, the optical gain mediumscapable of lasing may generate individual optical signals in differentband of wavelengths such as the O-band (around 1300 nm), S-band (around1480 nm), etc.

A second multiplexer/demultiplexer in the optical receiver array 554routes individual signals in the C-band to each of the correspondingoptical receivers. Each of the optical receivers receives a separatesignal containing a band of wavelengths different than the other opticalreceivers in the array 554. The array of optical gain mediums capable oflasing 550 and the array of optical receivers 554 may be positioned atset angles such as approximately 90 degrees and approximately 180degrees with respect to the band splitting filter 560 to route opticalsignals with waveguides, lenses or in air and without using opticalfibers.

FIG. 6 illustrates a block diagram of an embodiment of an array oflasers and an array of optical receivers in an integral unit. The arrayof lasers 650 and the array of receivers 664 may each couple to a lensarray 670, 672. The integral unit 676 may contain the array of lasers650, a band splitting filter 660, a multiplexer/demultiplexer 674, thearray of optical receivers 664, a first lens array 670, a second lensarray 672, and a connection 610 to a passive optical network. Themultiplexer/demultiplexer 674 routes signals to and from the array ofreceivers 664 and the array of lasers 650. The multiplexer/demultiplexer674 routes signals back and forth from the passive optical network. Thearray of optical receivers 664 is put close to themultiplexer/demultiplexer 674 so that the angle of the reflected C-bandwavelengths from the band splitting filter 660 remains at a small angle.The array of optical receivers 664 may be positioned at set angles suchas 45 degrees or less to route optical signals with air and the secondlens array 672. The band splitting filter 660 coated with a standarddielectric coating may split different wavelength, bands such as theC-band wavelengths and the L-band wavelengths.

The fabrication of the array of optical receivers and lasers in a singleintegral unit may be accomplished in a more simple fashion by using aband-splitting filter reflecting optical signals to the arrays at asmall angle. If the laser array is operated in the O-band, around 1300nanometers, rather than the L-band, band, then the reflected angle maybe about 90° which can make the packaging easier by using a beamsplitting prism. All of the configurations described above may beintegrated into a passive optical network. The passive optical networkmay or may not wavelength lock gain mediums capable of lasing byinjecting a narrow band Amplified Spontaneous Emission light into laserdiodes acting as gain mediums capable of lasing.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset fourth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustration rather then a restrictivesense.

1. An apparatus, comprising: an array of optical gain mediums that eachsupply a different wavelength; an array of optical receivers that eachreceive a different wavelength; a band splitting filter coupled to thearray of optical gain mediums and the array of optical receivers,wherein the array of the optical gain mediums and the array of opticalreceivers are positioned at opposite sides of the band splitting filter,wherein optical signals are reflected from the band splitting filter tothe array of optical receivers at an angle smaller than a predeterminedangle.
 2. An apparatus as in claim 1, further comprising an opticalrouting device coupled to the band splitting filter, wherein the opticalrouting device and the array of optical receivers are positioned at thesame side of the band splitting filter.
 3. An apparatus as in claim 1,wherein the array of optical gain mediums is grown on a first substrateand the array of optical receivers is grown on a second substrate.
 4. Anapparatus, comprising: an array of optical gain mediums that each supplya different wavelength; an array of optical receivers that each receivea different wavelength; a band splitting filter coupled to the array ofoptical gain mediums and the array of optical receivers, wherein thearray of the optical gain mediums and the array of optical receivers arepositioned at opposite sides of the band splitting filter; and abroadband light source to supply an optical signal containing a firstbroad band of wavelengths to the optical routing device, wherein atleast one of the optical gain mediums to couple to its own port of theoptical routing device to receive a spectral slice of the optical signalfrom the broadband light source to wavelength lock an output wavelengthof the at least one of the optical gain mediums within a bandwidth ofthe spectral slice.
 5. An apparatus as in claim 1, wherein the array ofoptical gain mediums includes a distributed feed back laser.
 6. Anapparatus as in claim 1, wherein the array of optical gain mediumsincludes a reflective semiconductor optical amplifier.
 7. An apparatusas in claim 1, wherein the array of optical gain mediums includes aFabry-Perot laser diode.
 8. An apparatus as in claim 1, furthercomprising a first array of optical routing devices coupled to the arrayof optical gain mediums.
 9. An apparatus as in claim 1, wherein thearray of optical gain mediums is operated in an O-band.
 10. An apparatusas in claim 2, wherein the array of optical gain mediums, the array ofoptical receivers, the band splitting filter and the optical routingdevice are integrated in an integral unit.
 11. An apparatus, comprising:an array of optical gain mediums that each supply a differentwavelength; an array of optical receivers that each receive a differentwavelength; a band splitting filter coupled to the array of optical gainmediums and the array of optical receivers, wherein the array of theoptical gain mediums and the array of optical receivers are positionedat opposite sides of the band splitting filter; and a connection to apassive optical network, wherein the connection is to route back aportion of first bands of wavelengths to at least one of the opticalgain mediums.
 12. A method, comprising: supplying first bands ofwavelengths from an array of optical gain mediums, wherein an opticalgain medium in the array supplies a band of wavelengths different thanthe other gain mediums in the array; multiplexing the first bands ofwavelengths to provide an output optical signal; demultiplexing secondbands of wavelengths routed from a passive optical network; andreflecting the demultiplexed second bands of wavelengths by a bandsplitting filter to an array of optical receivers.
 13. A method as inclaim 12, further comprising reflecting a portion of the first bands ofwavelengths through a multiplexer/demultiplexer to at least one of theoptical gain mediums.
 14. A method as in claim 12, wherein thedemultiplexed second bands of wavelengths are reflected by the bandsplitting angle at an angle smaller than a predetermined angle.
 15. Amethod as claim 12, further comprising routing the first bands ofwavelengths to a passive optical network.